This invention relates to method and apparatus for collecting X-ray absorption data in an X-ray computed tomography apparatus, which can obtain precise X-ray absorption data with respect to a given plane section of a patient's body and also permits reconstruction of a high quality tomographic image.
Among X-ray tomographic apparatus, there are computorized tomographic scanners (hereinafter referred to as CT scanner).
One such CT scanner adopts a traverse and rotational system also called a "first generation" system. FIG. 1 shows such a first generation system. In this system, an X-ray tube XT and an X-ray detector DT are disposed to face each other on either side of a patient's body P, and they are traversed together in the direction of the dot-dash arrows in FIG. 1, while the X-ray tube emits X-rays along a plane section of the body P. The x-ray tube XT and the X-ray detector DT being so traversed, a radiation path XR is obtained which is parallel to the plane section of the body P. After one cycle of such scanning is ended, the angle of incidence of X-radiation on the plane section of the body P is changed. The linear motion and rotational motion are alternately repeated as shown in FIG. 2 to collect X-ray absorption data usually over an angle range of 180 degrees. The reconstitution of every point of the plane section of the body P is possible by permitting X-rays to traverse the plane section substantially in all directions or angular aspects.
The X-ray absorption data obtained at the X-ray detector DT is analyzed in an electronic computer to calculate the X-ray absorption factor of individual points of the plane section. A tomographic image is reconstructed through the analysis of each point of the body's plane section on the basis of the determined X-ray absorption factor and with a graduation degree amounting to more than 2,000 steps. In addition to the first generation, there is another traverse and rotational system CT scanner sometime called a "second generation". As shown in FIG. 3, the second generation system uses as the X-ray source an X-ray tube XT which generates X-rays in the form of a sector having a small angle. An X-ray detector DT is disposed to face the X-ray tube XT on the opposite side of the body P as mentioned above. The X-ray detector DT has a plurality of X-ray detecting elements arranged side by side. For the collection of X-ray absorption data, the X-ray tube XT and X-ray detector DT are traversed along a plane section of the body P and, after the completion of traverse scanning in each cycle, rotated by a predetermined angle.
As has been shown, either in the first or second generation the reconstruction of the final image involves image formation through conversion of the value of X-ray absorption that is determined by the structure of the inspected body into a corresponding shade signal. The X-ray absorption factor, i.e., the value of each pixel constituting the image, is referred to as the "CT value". This CT value is linearly related to the X-ray absorption factor and is called the "Hounsfield number" after an early worker in CT technology. This value corresponds to an absorption factor value based on an X-ray energy level of 73 keV (hereinafter referred to as .mu. value). Construction of the final image is effected after deriving the CT value. However, the CT value is not an abosolutely definite value for the following reasons. The X-radiation from commercially available CT scanners usually does not consist of a single wavelength at the aforementioned reference energy level of 73 keV but has a spectral distribution. In addition, the X-ray absorption factor M of a predetermined structure of the body is not fixed because of variations of the proportions of the photoelectric effect and Compton effect. More specifically, the X-ray absorption in the neighborhood of a predetermined structure of the body varies depending upon whether or not the structure is surrounded by bones, and an energy spectrum shift, if ay, causes a deviation of the CT value. The X-ray energy distribution which has the most significant influence upon the X-ray absorption value, can be determined by varying the tube voltage of the X-ray tube.
Most existing CT scanners use only a single fixed tube voltage to achieve linear scanning of a plane section of the body. It is therefore necessary to scan the same plane section two or more times, moving the X-ray tube and X-ray detector repeatedly and applying a different voltage on the X-ray tube during each linear scan. This increases the X-ray exposure dose to the patient and requires a longer time, and hence represents a distinct drawback in the system.
It has been contemplated to provide a process for compensating for the shift of the CT value during the signal processing step performed during image construction. However, no correction method that is effective for all bodies and for all scanning conditions has been found.